State-of-the-art, high-efficiency blue phosphorescent and thermally-activated delayed fluorescence (TADF) organic light-emitting devices (OLEDs) exhibit operational lifetimes that are at least 20 and 45 times shorter than state-of-the-art, high-efficiency green and red OLEDs, respectively (the latter having L50 lifetimes of up to 900,000 h at 1000 cd/m2; device lifetime, L50, defined as the time to reach 50% of initial luminance). Thus, while organic electroluminescent devices employing green and red triplet-exciton-harvesting and triplet-exciton-emitting (i.e., phosphorescent) materials have met the requirements for various applications, including but not limited to modern-day lighting and display technologies, the development of stable, high-efficiency blue organic light-emitting devices is an ongoing challenge.
As a result, to-date, there are no commercial OLED lighting or display products that incorporate high-efficiency, blue phosphorescent or TADF emitters. (Y. Zhang, J. Lee, S. R. Forrest, Tenfold increase in the lifetime of blue phosphorescent organic light-emitting diodes. Nat. Commun. 5, 5008 (2014); S. Reineke Complementary LED technologies, Nat. Mater. 14, 459-462 (2015); J.-H. Jou, S. Kumar, A. Agrawal, T.-H. Li, S. Sahoo, Approaches for fabricating high efficiency organic light emitting diodes. J. Mater. Chem. C, 3, 2974-3002 (2015)). Existing commercial OLED devices predominantly employ blue fluorescent materials with low internal quantum efficiencies (˜25% due to the lack of triplet exciton harvesting), but reasonably long device lifetime (L50 up to 50,000 h at 1000 cd/m2 reported for blue fluorescent OLEDs in commercial displays). (R. Mertens, The OLED Handbook (2015), publisher: Ron Mertens; J.-H. Jou, S. Kumar, A. Agrawal, T.-H. Li, S. Sahoo, Approaches for fabricating high efficiency organic light emitting diodes. J. Mater. Chem. C, 3, 2974-3002 (2015); H. Kuma, C. Hosokawa, Blue fluorescent OLED materials and their application for high performance devices. Sci. Technol. Adv. Mater. 15, 034201-1-7 (2014)). It has been predicted that should stable, high-efficiency blue phosphorescent OLEDs be added to OLED display technologies, a 30% savings in power consumption could be realized. This represents a significant energy savings to the consumer and could substantially improve the energy efficiency of lighting and display technologies, in addition to improving their operational lifespan. (OLED-info, http://www.oled-info.com/blue-pholed-breakthrough-researchers-manageextend-lifetime-tenfold, accessed July 2017).
Maintaining both long-term stability and high efficiency has been problematic due to the long electronic excited-state lifetimes of phosphorescent materials and particularly so for blue emitters due to their high energy (i.e., large bandgap) electronic excited states which facilitate multiple undesirable non-radiative decay pathways. Furthermore, achieving deep-blue or true blue emission (430-465 nm wavelength range) with Commission Internationale de L'Eclairage coordinates, CIE (x,y), of (0.14, 0.08) has proven to be a significant challenge. (Y. Zhang, J. Lee, S. R. Forrest, Tenfold increase in the lifetime of blue phosphorescent organic light-emitting diodes. Nat. Commun. 5, 5008 (2014); H. Kuma, C. Hosokawa, Blue fluorescent OLED materials and their application for highperformance devices. Sci. Technol. Adv. Mater. 15, 034201-1-7 (2014); T. Sajoto, P. I. Djurovich, A. B. Tamayo, J. Oxgaard, Temperature Dependence of Blue Phosphorescent Cyclometalated Ir(III) Complexes, J. Am. Chem. Soc. 131, 9813-9822 (2009); Y. J. Cho, K. S. Yook, J. Y. Lee, Cool and warm hybrid white organic light-emitting diode with blue delayed fluorescent emitter both as blue emitter and triplet host. Sci. Rep. 5, 7859-1-7 (2015); H. Fu, Y.-M. Cheng, P.-T. Chou, Y. Chi, Feeling blue? Blue phosphors for OLEDs. Mater. Today, 14, 472-479 (2011)). Due to the low stability of blue phosphors, lighting applications have been particularly impeded due to the high power efficiencies required for lighting applications compared to display applications.
In organic light-emitting materials, the yield of singlet excitons to triplet excitons under optical excitation is usually different to that under electrical excitation. Under optical excitation of singlets there is little or no conversion to the triplet state due to strong spatial localization and the large exciton binging energy of the Frenkel-type singlet excitons and, hence, their large exchange energy. (A. P. Monkman, Singlet Generation from Triplet Excitons in Fluorescent Organic Light-Emitting Diodes. ISRN Mater. Sci. 670130 (2013)). Under electrical excitation, electron and hole polarons are formed that are initially uncorrelated—i.e., their spin states are randomly oriented with respect to one another. Only at the point of recombination do they become correlated and both singlet and triplet excitons form. If it is assumed that recombination is spin independent, 75% or excitons that are formed are triplets and 25% are singlets. This has been demonstrated to be the case particularly for small organic conjugated molecular systems. (J. Wang, A. Chepelianskii, F. Gao, N. C. Greenham, Control of exciton spin statistics through spin polarization in organic optoelectronic devices. Nature Commun. 3, 1191 (2012); I. Bergenti, V. Dediu, E. Arisi, T. Mertelj, M. Murgia, A. Riminucci, G. Ruani, M. Solzi, C. Taliani, Spin polarised electrodes for organic light emitting diodes. Org. Electron. 5, 309-314 (2004); J. S. Wilson, A. S. Dhoot, A. J. A. B. Seeley, M. S. Khan, A. Kohler, R. H. Friend, Spindependent exciton formation in pi-conjugated compounds. Nature 413, 828-831 (2001); A. Köhler, H. Bässler, Triplet states in organic semiconductors. Mater. Sci. Eng. R 66, 71-109 (2009); Y. Zhang, M. Whited, M. E. Thompson, S. R. Forrest, Singlet-triplet quenching in high intensity fluorescent organic light emitting diodes. Chem. Phys. Lett. 495, 161-165 (2010)). Since triplet exciton radiative decay is a forbidden transition, for triplets to emit light, organometallic molecules (i.e., “triplet-harvesting emitter materials”; typically iridium-based organometallic complexes) that facilitate triplet radiative decay either by spin-orbit coupling due to the “heavy” atom of the transition metal or by TADF are incorporated into the emissive layer. (S. Kappaun, C. Slugovc, E. J. W. List, Phosphorescent Organic Light-Emitting Devices: Working Principle and Iridium Based Emitter Materials. Int. J. Mol. Sci. 9, 1527-1547 (2008); H. Uoyama, K. Goushi, K. Shizu, H. Nomura, C. Adachi, Highly efficient organic light emitting diodes from delayed fluorescence. Nature 492, 234-238 (2012)). These dopants accelerate intersystem crossing from excited singlet to light-emitting triplet states (and vice versa in the case of TADF) and thus significantly improve the internal quantum efficiency. As a result, close to 100% of excitons electrically excited can decay radiatively and emit light, thereby yielding high-efficiency OLED devices.
For high-efficiency, phosphorescent emitters at low luminances, the radiative lifetime (σTr) is faster than the non-radiative lifetimes (σTnr). However, for blue organic phosphorescent materials, in particular, it has been shown that an additional fast non-radiative decay component (triplet quenching) is introduced due to defect formation when triplet-polaron annihilation (TPA) and triplet-triplet annihilation (TTA) occur (πnrQ). (N. C. Giebink, B. W. D'Andrade, M. S. Weaver, P. B. Mackenzie, J. J. Brown, M. E., Thompson, S. R. Forrest, Intrinsic luminance loss in phosphorescent small-molecule organic light emitting devices due to bimolecular annihilation reactions. J. Appl. Phys. 103, 044509 (2008); Y. Zhang, J. Lee, S. R. Forrest, Tenfold increase in the lifetime of blue phosphorescent organic light-emitting diodes. Nat. Commun. 5, 5008 (2014); N. C. Giebink, B. W. D'Andrade, M. S. Weaver, J. J. Brown, S. R. Forrest, Direct evidence for degradation of polaron excited states in organic light emitting diodes. J. Appl. Phys. 105, 124514 (2009); R. Coehoorn, H. van Eersel, P. A. Bobbert, R. A. J. Janssen, Kinetic Monte Carlo Study of the Sensitivity of OLED Efficiency and Lifetime to Materials Parameters. Adv. Funct. Mater. 25, 2024-2037 (2015)). This is the main cause of the short blue operational lifetimes in high-efficiency blue phosphorescent OLEDs (note that operational lifetimes quantify the stability of the OLED devices; radiative and non-radiative lifetimes quantify how fast excited electrons recombine with holes in a ground electronic state). Further degradation of high-efficiency blue phosphorescent emitters (i.e., triplet exciton emitters) occurs at high luminance due to accumulations of non-radiative recombination centers and quenchers in the light emitting zone. (R. Coehoorn, H. van Eersel, P. A. Bobbert, R. A. J. Janssen, Kinetic Monte Carlo Study of the Sensitivity of OLED Efficiency and Lifetime to Materials Parameters. Adv. Funct. Mater. 25, 2024-2037 (2015); D. Y. Kondakov, W. C. Lenhart, W. F. Nichols, Operational degradation of organic light emitting diodes: Mechanism and identification of chemical products, J. Appl. Phys. 101, 024512 (2007); R. Seifert, I. R. de Moraes, S. Scholz, M. C. Gather, B. Lüssem, K. Leo, Chemical degradation mechanisms of highly efficient blue phosphorescent emitters used for organic light emitting diodes. Org. Electron. 14, 115-123 (2013)). Besides TPA and TTA, other factors that contribute to triplet quenching are: (1) field-induced exciton dissociation; (2) loss of charge balance; and (3) triplet oxygen quenching. (N. C. Giebink, B. W. D'Andrade, M. S. Weaver, P. B. Mackenzie, J. J. Brown, M. E. Thompson, S. R. Forrest, Intrinsic luminance loss in phosphorescent small-molecule organic light emitting devices due to bimolecular annihilation reactions. J. Appl. Phys. 103, 044509 (2008)). Due to these stability issues associated with blue organic phosphorescent emitters, all commercial OLED displays currently use low-efficiency blue fluorescent emitters, which limits their energy conversion efficiencies (i.e., electrical-to-light conversion efficiencies). The more stable phosphorescent true-blue emitters exhibit device operational lifetimes of a few thousand hours at best, far short of the ˜10,000 h required for commercial viability. (N. C. Giebink, B. W. D'Andrade, M. S. Weaver, P. B. Mackenzie, J. J. Brown, M. E. Thompson, S. R. Forrest, Intrinsic luminance loss in phosphorescent small-molecule organic light emitting devices due to bimolecular annihilation reactions. J. Appl. Phys. 103, 044509 (2008); Y. Zhang, J. Lee, S. R. Forrest, Tenfold increase in the lifetime of blue phosphorescent organic light-emitting diodes. Nat. Commun. 5, 5008 (2014); R. Mertens, The OLED Handbook (2015), publisher: Ron Mertens; M. Kim, S. K. Jeon, S.-H. Hwang, J. Yeob Lee, Stable Blue Thermally Activated Delayed Fluorescent Organic Light-Emitting Diodes with Three Times Longer Lifetime than Phosphorescent Organic Light-Emitting Diodes, Adv. Mater. 27, 2515-2520 (2015)). Recently, Forrest and co-workers extended the lifetime of blue phosphorescent OLEDs by a factor of 10 to ˜3,500 h (L50) at 1000 cd/m2 by grading the emissive dopant concentration profile in the emissive layer which led to lower exciton densities and, hence, lower TPA compared to conventional devices. (Y. Zhang, J. Lee, S. R. Forrest, Tenfold increase in the lifetime of blue phosphorescent organic light-emitting diodes. Nat. Commun. 5, 5008 (2014)). Furthermore, it is notable that, while TTA degrades the performance of phosphorescence OLEDs, TTA can improve the performance of fluorescent OLED devices mainly because, of the ˜75% of triplets that form upon electrical excitation of fluorescent materials, pairs of triplets can annihilate to form a higher energy singlet exciton. Thus, theoretically increasing the yield of singlet exciton emission to 62.5% [25%+(75%/2)]. (C. Ganzorig, M. Fujihira, A possible mechanism for enhanced electrofluorescence emission through triplet-triplet annihilation in organic electroluminescent devices. Appl. Phys. Lett. 81, 3137 (2002); D. Y. Kondakov, T. D. Pawlik, T. K. Hatwar, J. P. Spindler, Triplet annihilation exceeding spin statistical limit in highly efficient fluorescent organic light-emitting diodes. J. Appl. Phys. 106, 124510 (2009); C. Mayr, T. D. Schmidt, W. Brutting, High-efficiency fluorescent organic light-emitting diodes enabled by triplet-triplet annihilation and horizontal emitter orientation. Appl. Phys. Lett. 105, 183304 (2014); H. Fukagawa, T. Shimizu, N. Ohbe, S. Tokito, K. Tokumaru, H. Fujikake, Anthracene derivatives as efficient emitting hosts for blue organic light-emitting diodes utilizing triplet—triplet annihilation. Org. Electron. 13, 1197-1203 (2012)). However, the stability of such TTA processes have yet to be determined.
As an alternative to rare-earth or noble-metal-containing organometallic phosphors that harvest triplet emission by spin-orbit coupling, metal-free organic molecules that exhibit TADF have been developed as an approach to achieving high-efficiency emitters for OLED applications that harvest both singlet and triplet excitons. (H. Uoyama, K. Goushi, K. Shizu, H. Nomura, C. Adachi, Highly efficient organic lightemitting diodes from delayed fluorescence. Nature 492, 234-238 (2012); Q. Zhang, B. Li, S. Huang, H. Nomura, H. Tanaka, C. Adachi, Efficient blue organic light emitting diodes employing thermally activated delayed fluorescence. Nat. Photon. 8, 326-332 (2014); H. Nakanotani, T. Higuchi, T. Furukawa, K. Masui, K. Morimoto, M. Numata, H. Tanaka, Y. Sagara, T Yasuda, C. Adachi, High-efficiency organic light-emitting diodes with fluorescent emitters. Nat. Commun. 5, 4016-1-7 (2014); H. Nakanotani, K. Masui, J. Nishide, T. Shibata, C. Adachi, Promising operational stability of high-efficiency organic light-emitting diodes based on thermally activated delayed fluorescence. Sci. Rep. 3, 2127-1-5 (2013)). Molecules that emit by TADF have small energy differences between singlet and triplet excited electronic states, thereby allowing efficient spin conversion without needing heavy metal atoms. While high internal quantum efficiency is obtainable using TADF, radiative lifetimes of high-efficiency blue TADF molecules are similar to those of high-efficiency blue organometallic phosphors (1-3 μs) because short triplet lifetimes compete with reverse inter-system crossing which is necessary for efficient TADF. (H. Uoyama, K. Goushi, K. Shizu, H. Nomura, C. Adachi, Highly efficient organic lightemitting diodes from delayed fluorescence. Nature 492, 234-238 (2012); Q. Zhang, B. Li, S. Huang, H. Nomura, H. Tanaka, C. Adachi, Efficient blue organic light emitting diodes employing thermally activated delayed fluorescence. Nat. Photon. 8, 326-332 (2014)). Promising operational lifetimes of up to 2,800 h (at 1000 cd/m2) and 52 h (at 500 cd/m2) have been reported at yellow and blue wavelengths, respectively, for TADF OLEDs. (H. Nakanotani, T. Higuchi, T. Furukawa, K. Masui, K. Morimoto, M. Numata, H. Tanaka, Y. Sagara, T Yasuda, C. Adachi, High-efficiency organic light-emitting diodes with fluorescent emitters. Nat. Commun. 5, 4016-1-7 (2014); H. Nakanotani, K. Masui, J. Nishide, T. Shibata, C. Adachi, Promising operational stability of high-efficiency organic light-emitting diodes based on thermally activated delayed fluorescence. Sci. Rep. 3, 2127-1-5 (2013); M. Kim, S. K. Jeon, S.-H. Hwang, J. Y. Lee, Stable Blue Thermally Activated Delayed Fluorescent Organic Light-Emitting Diodes with Three Times Longer Lifetime than Phosphorescent Organic Light-Emitting Diodes. Adv. Mater. 27, 2515-2520 (2015)). However, it is yet to be determined if the stabilities of TADF OLEDs can improve upon those of state-of-the-art phosphorescent OLEDs.
Fermi's Golden Rule states that the spontaneous emission rate (i.e., the radiative decay rate, Γr, (=σr−1)) is proportional to two factors: (1) the transition dipole moment of an emitting dipole; and (2) the surrounding local density of optical states (LDOS). The former is an intrinsic property of the molecule, while the latter is an extrinsic property of the optical environment in which the molecules is situated.
Current technologies are focused solely on intrinsic methods for increasing stability. Various approaches have been investigated, yet high-efficiency blue OLED operational lifetimes are still too short for commercial viability. Thus, an extrinsic method for improving stability is beneficial and needed.